Is That Why They Call It a "Head" of Lettuce?

I've always been curious about the fact (claim) that humans and chimpanzees have genetic material that is 95-98 percent identical. I don't know what that means (it seems like we are more than 2-5% different from these primates, opposable thumbs notwithstanding) but I was interested in this fact: The very high correlation between human and chimpanzee gene sequences is "less meaningful than it sounds. Humans share more than 80 percent of their gene sequence with mice, and maybe 40 percent with lettuce" (National Geographic, "Almost Human," by Mary Roach, April 2008, p. 134).

Now I'm curious as to which 40% of me is identical to lettuce. I'm pretty sure that somewhere in the 60% difference is the fact that I can type.